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Attractive and repulsive Fermi polarons in two dimensions


The dynamics of a single impurity in an environment is a fundamental problem in many-body physics. In the solid state, a well known case is an impurity coupled to a bosonic bath (such as lattice vibrations); the impurity and its accompanying lattice distortion form a new entity, a polaron. This quasiparticle plays an important role in the spectral function of high-transition-temperature superconductors, as well as in colossal magnetoresistance in manganites1. For impurities in a fermionic bath, studies have considered heavy or immobile impurities which exhibit Anderson’s orthogonality catastrophe2 and the Kondo effect3. More recently, mobile impurities have moved into the focus of research, and they have been found to form new quasiparticles known as Fermi polarons4,5,6,7. The Fermi polaron problem constitutes the extreme, but conceptually simple, limit of two important quantum many-body problems: the crossover between a molecular Bose–Einstein condensate and a superfluid with BCS (Bardeen–Cooper–Schrieffer) pairing with spin-imbalance8 for attractive interactions, and Stoner’s itinerant ferromagnetism9 for repulsive interactions. It has been proposed that such quantum phases (and other elusive exotic states) might become realizable in Fermi gases confined to two dimensions10,11. Their stability and observability are intimately related to the theoretically debated12,13,14,15,16 properties of the Fermi polaron in a two-dimensional Fermi gas. Here we create and investigate Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas, measuring their spectral function using momentum-resolved photoemission spectroscopy17,18,19. For attractive interactions, we find evidence for a disputed pairing transition between polarons and tightly bound dimers, which provides insight into the elementary pairing mechanism of imbalanced, strongly coupled two-dimensional Fermi gases. Additionally, for repulsive interactions, we study novel quasiparticles—repulsive polarons—the lifetime of which determines the possibility of stabilizing repulsively interacting Fermi systems.

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Figure 1: Polaron energy level diagram and measured single particle spectral function.
Figure 2: Attractive polaron.
Figure 3: Finite temperature and impurity concentration.
Figure 4: Repulsive polaron.


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We thank S. Baur, N. Cooper, E. Demler, T. Enss, C. Kollath, J. Levinsen, M. Parish, R. Schmidt and W. Zwerger for discussions, and J. Bohn for communicating unpublished details of the |−5/2〉/|−3/2〉 Feshbach resonance in 40K. The work was supported by EPSRC (EP/G029547/1), Daimler-Benz Foundation (B.F.), Studienstiftung and DAAD (M.F.).

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The experimental set-up was devised and constructed by M.F., B.F., E.V. and M. Köhl, data taking was performed by M. Koschorreck, E.V. and D.P., data analysis was performed by M. Koschorreck and D.P., and the manuscript was written by M. Köhl with contributions from all co-authors.

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Correspondence to Michael Köhl.

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Koschorreck, M., Pertot, D., Vogt, E. et al. Attractive and repulsive Fermi polarons in two dimensions. Nature 485, 619–622 (2012).

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